1. Field of the Invention
The present invention relates to a pulsation reducing apparatus that reduces pressure pulsation, which is generated and conducted in a fuel passage due to operation of a fuel pump.
2. Description of Related Art
For instance, a known common rail fuel injection system includes an accumulator (a common rail) that supplies high pressure fuel to fuel injection valves installed to cylinders of a diesel engine of a vehicle. The fuel injection system further includes a low pressure pump and a high pressure pump. The low pressure pump pumps fuel out of a fuel tank. The high pressure pump pressurizes and discharges the fuel, which is supplied from the low pressure pump, to the common rail. Normally, a filter is provided in the fuel passage, which connects between the fuel tank and the low pressure pump, to filter contaminants contained in the fuel.
Lately, it is often reported from users that a quality of the fuel is deteriorated (more specifically, for example, the amount of contaminants in the fuel being increased), thereby causing clogging of the filter. When the clogging of the filter occurs, the fuel pressure on the downstream side of the filter is substantially reduced (thereby resulting in an increase in a negative pressure). When the negative pressure is increased, air bubbles may possibly be generated in the fuel. When the air bubbles are pressurized by the high pressure pump along with the fuel, the quality of the fuel may possibly be deteriorated due to an increase in the fuel temperature. In order to avoid such a disadvantage, it is desirable to replace the filter upon occurrence of the clogging of the filter. However, due to the deterioration of the quality of the fuel, the life of the filter is disadvantageously shortened, so that the frequency of the filter replacement is disadvantageously increased. It is conceivable to increase a pore size (also referred to as a mesh size) of the filter to avoid the clogging of the filter. However, the increase in the pore size of the filter disadvantageously results in a reduction in a filtration efficiency of the filter.
In view of the above disadvantages, a common rail fuel injection system, which includes a prefilter and a main filter, has been developed. The prefilter is placed in a fuel passage, which connects between the fuel tank and the low pressure pump. The prefilter has a large pore size. The main filter is placed in a fuel passage, which connects between a fuel outlet of the low pressure pump and a fuel inlet of the high pressure pump. The main filter has a pore size, which is smaller than that of the prefilter. This type of common rail fuel injection system has been put into practical use. Specifically, the fuel, from which the contaminants are removed by the prefilter, is supplied to the main filter by the low pressure pump. Then, the fuel, from which the contaminants are further removed by the main filter, is supplied to the high pressure pump.
The pressure pulsation (also referred to as pressure fluctuation, pressure oscillation or pressure surge) of the fuel is generated in the fuel pump due to the operation of the low pressure pump. In a case of a fuel injection system, which is installed in a construction machine (e.g., an excavator), the fuel tank may possibly be refueled in an environment where contaminants (e.g., dust) are present in the large amount and are floating in the air as the dust blast, there is a high possibility that the contaminants are supplied into the fuel tank along with the fuel. Therefore, even in the case of the above-discussed fuel injection system, which includes the prefilter having the large pore size, the clogging of the prefilter may possibly occur. In such a case, as discussed above, the air bubbles may be generated in the fuel due to the increase in the negative pressure on the downstream side of the prefilter. When the air bubbles are drawn into and compressed in the low pressure pump along with the fuel, the pressure pulsation may possibly be abnormally increased. Furthermore, in a case where the excavator is operated at a highland (high altitude), the air bubbles can be easily mixed into the fuel due to the low atmospheric pressure. The inclusion of the air bubbles in the fuel may possibly cause the abnormally increased pressure pulsation. Furthermore, in a case where the inclusion of the air bubbles into the fuel is likely to occur due to a reduction in the airtightness of the fuel passage on the upstream side of the low pressure pump, the pressure pulsation may possibly be abnormally further increased. In the case where the pressure pulsation is abnormally increased, when such a pressure pulsation is conducted to the main filter, the main filter may possibly be damaged.
In order to avoid the above disadvantage, it has been proposed to place a damper in the fuel passage, which connects between the low pressure pump and the main filter and/or to use a rubber conduit to form the fuel passage. Japanese Unexamined Patent Publication No. H06-101793A teaches an active pulsation reducing apparatus, which addresses the above disadvantage and uses a piezoelectric element that is driven based on a measurement signal of a pressure sensor, which indicates pressure pulsation in a fluid conduit, to drive a piston placed in a casing communicated with the interior of the fluid conduit and thereby to provide a counteracting pressure for the pressure pulsation.
However, the countermeasures, such as the provision of the damper and the use of the rubber conduit, can reduce only the small amount of the abnormally increased pressure pulsation. Therefore, the reliability of the main filter may possibly be deteriorated. In the case of the Japanese Unexamined Patent Publication No. H06-101793A, the costs may possibly be disadvantageously increased due to the complex structure of the active pulsation reducing apparatus.
Thus, there is a substantial need for improving the technique in order to reduce the abnormal pressure pulsation.
The disadvantage is not limited to the system having the main filter. Specifically, in the case of the fuel supply system of the internal combustion engine that includes the fuel pump, which pumps the fuel from the fuel tank, and the predetermined fuel receiving subject device, to which the fuel is supplied from the fuel pump through the fuel passage, the function of the predetermined fuel receiving subject device may possibly be damaged in the case where the pressure pulsation is generated due to the operation of the fuel pump and is conducted through the fuel passage.